The virological factors, both biological and molecular, affecting the pathogenicity of murine C-type viruses of the ecotropic and recombinant MCF virus classes are under study. In general, the MCF viruses which are thymus-derived are also thymotropic and lymphomagenic and have certain T1 oligonucleotide patterns in common; MCF virus obtained from other sources do not replicate in thymus, do not induce or accelerate lymphoma, and possess a different class of distinguishing oligonucleotide patterns. Biological characterization is in progress of viruses which are derived by transfection of DNA recombinants contructed from restriction enzyme fragments of cloned DNA of a lymphomagenic MCF virus and of AKR ecotropic virus. These viruses are being tested for host range, tissue culture plating efficiency, lymphomagencity, in vivo replication, etc. in attempts to identify biological activities controlled by specific viral gene regions. Ecotropic viruses which are leukemogenic all replicate efficiently in vivo and regularly induce formation of MCF viruses; certain of these viruses are also being studied for the effects of interchanging portions of the viral genomes.